Every year in the United States, approximately 150,000 people die from acute lung injury (ALl). Bacterial lipopolysaccharide (LPS) released into the bloodstream of patients from a source of infection is an important contributor to the endothelial dysfunction observed in patients with acute lung injury. We have shown that the cell death pathway induced by LPS in primary human lung microvascular endothelial cells requires inhibition of transcription and induction of the mitochondrial-dependent cell death pathway. In this proposal, we have developed two interrelated specific aims to determine the mechanism(s) by which exposure of the cell to LPS activates the cell death pathway. Specifically we will determine (1) whether cell death following exposure to LPS requires activation of caspase-8 and truncation of Bid through a FADD dependent pathway and (2) whether the MAPK pathway p38 is required for LPS induced cell death upstream of caspase activation. All of the experiments in this proposal will be performed using genetic strategies in primary human lung microvascular endothelial cells. By elucidating the cell death mechanisms activated by exposure to LPS, we hope to facilitate the development of rational strategies for the treatment of patients at risk for ALl.